New Holland 3904 – Engine Control Unit (ECU) / EDC: Fuel Temperature Sensor: failed plausibility check at ECU start

When the engine is cold and the ignition is first turned on, the Engine Control Unit (ECU) runs a plausibility check by comparing the readings of various temperature sensors (such as the fuel temperature, coolant temperature, and intake manifold temperature). If the fuel temperature sensor reports a value that is drastically different from the other sensors, the ECU determines the reading is implausible and logs fault code 3904. This typically indicates a failing sensor, a wiring issue, or an external factor (like a block heater) skewing the temperature readings.

• T6
• T7
• T6000
• T7000
• TG
• TJ
• B90B
• B95B
• B95C
• B110B
• B115B
• L225
• L228
• L230
• C232
• C238
• E235BSR
• CR6.80
• CR7.90
• CR9060

1. 1

   Perform a cold soak comparison: Connect a diagnostic tool after the machine has sat overnight. Compare the Fuel Temperature, Coolant Temperature, and Intake Air Temperature. They should all be within a few degrees of each other. If fuel temperature is drastically off (e.g., -40°C or 150°C), the sensor or circuit is faulty.

2. 2

   Inspect the fuel temperature sensor (usually mounted on the primary/secondary fuel filter housing or fuel manifold), its 2-pin electrical connector, and the wiring harness for chafing, corrosion, or loose pins.

3. 3

   Perform a resistance test: Disconnect the sensor and measure its resistance with a multimeter. Compare the reading against the OEM specification for the current ambient temperature to confirm an open or short circuit.

4. 4

   Check for block heater interference: Ensure that an engine block heater or external heat source hasn't artificially heated the coolant while leaving the fuel cold, which can trigger a false plausibility failure.

1. 1

   Preparation

   Turn off the ignition, remove the key, and disconnect the negative battery cable to prevent electrical spikes.

2. 2

   Locate the Sensor

   Find the fuel temperature sensor on the fuel filter base or fuel line manifold. Clean the area around the sensor to prevent dirt from entering the fuel system.

3. 3

   Removal

   Disconnect the electrical harness plug from the sensor. Place a rag or small catch pan underneath to catch the small amount of diesel fuel that will leak out. Use a wrench (typically 19mm) to unscrew and remove the sensor.

4. 4

   Installation

   Thread the new sensor into the housing by hand to prevent cross-threading. Tighten it to the OEM torque specification using a wrench.

5. 5

   Reconnection

   Reconnect the electrical harness, ensuring the weather-tight seal clicks into place.

6. 6

   System Bleeding

   If a significant amount of fuel was lost, you may need to use the hand primer pump on the fuel filter assembly to bleed air from the low-pressure side of the fuel system.

7. 7

   Clear Codes

   Reconnect the battery. Turn the ignition on and use your diagnostic tool to clear the inactive 3904 fault code. Start the engine and verify there are no fuel leaks around the new sensor.